Search directive feature for FSI

It would be cool if FSI had type signature search for apis in the loaded assemblies, like Idris.

```idris
Idris> :search String -> Nat
= Prelude.Strings.length : String -> Nat
Returns the length of the string.

< idris_crash : String -> a
Abort immediately with an error message

< Prelude.Cast.cast : Cast from to => from -> to
Perform a (potentially lossy!) cast operation.
```

In F# it can be something like:

```fsharp
Microsoft (R) F# Interactive version 11.0.0.0 for F# 5.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> #search string -> int
Int32.Parse : string -> int
String.length : string -> int
MyModule.myCast : string -> 'a
```